In situations of great vertical wind shear, a strong thunderstorm updraft can interact with the surrounding flow in direct, profound ways.

One of these interactions, discussed in class, is the formation of the feature known as the Rear Flank Downdraft.

Just as strong winds encountering a solid obstacle, such as a telephone pole, can generate small eddies, or whirlpools, downwind of the obstacle, the vertical shear (which is in turn related to the jet stream) can create disturbances on the flanks of thunderstorms.

These disturbances are very small in diameter, but can act to either augment or suppress the updrafts.

In the case of the vertical shear that is favorable for supercell thunderstorms, in some patterns the updraft is continually reinforced on its southeast side, and weakened on its northwest side. So, as the thunderstorm moves along, to the observer, it appears that the thunderstorm "turns to the right".

On radar, this is very distinctive. While other thunderstorms move with the average flow, say, southwesterly, at 500 mb, supercells developing in this kind of environment can appear to move towards the east or southeast, or even become stationary. Such storms are said to deviate from the motion expected given the average winds and are called "deviate movers".

Meteorologists look for deviate motion as an indication that the storm might be a supercell.

Deviate Supercell -- Actually stationary near Medicine Lodge, KS, 5/13/04 -- Multiple Tornadoes (Note also Hook Echo)

Deviate Supercell --Moving East-Northeastward near Hill City, KS -- Multiple Tornadoes (Note also Hook Echo)

Outbreak of Deviate Supercells --Moving East-Northeastward -- Multiple Tornadoes, including 4 F5 (Note monstrous hook echoes)